Methods of construction of evacuated glazing

Abstract: A thermally insulating glass panel [1'], comprising two spaced apart sheets of glass [2, 3] enclosing a low pressure space [4] and interconnected by a glass solder edge seal [5] and an array of support pillars, and further comprising a pump-out tube [6'] which penetrates one of the glass sheets, characterized in that the pump-out tube [6'] has a radial flange [10] which is cemented to the inner surface of that glass sheet. A method of constructing a thermally insulating glass panel [1'] comprising two spaced apart sheets of glass [2, 3] enclosing a low pressure space [4] interconnected by a solder glass edge seal [5] and an array of pillars, the method including the step of installing a pump-out tube [6'] through one of the glass sheets, characterized in that the pump-out tube [6'] has a radial flange [10] which is sealed to the interior surface of the glass sheet.

Description: Introduction The invention described in this specification concerns an improvement to the design of thermally insulating glass panels. These panels typically comprise two spaced apart sheets of glass enclosing a low pressure space and interconnected by an edge seal and an array of support pillars. The panels are used for glazing, and such thermally insulating windows are generally referred to as evacuated glazing. Background of the Invention Thermally insulating panels are typically constructed by depositing a strip of solder glass around the periphery of the glass sheets; depositing an array of support pillars onto one or other of the sheets of glass; bringing the sheets together or permitting them to move together; heating the panels to melt the solder glass around the periphery so that the two sheets settle onto the pillars; and then cooling the panel to solidify the edge seal. The panel is then evacuated by pumping out through a tube positioned either through one of the glass sheets or through the edge seal, and finally the pump-out tube is melted and sealed off. The invention concerns a panel incorporating a pump-out tube, and a method of constructing such panels. Several conditions must be satisfied in the construction of evacuated glazing. First, it is necessary to produce an hermetic (leak-free) edge seal around the periphery of the panel. Second, in order to prevent the glass sheets from being forced into contact with each other, by the large atmospheric pressure forces acting, it is essential to include an array of support pillars within the glazing. Third, the space between the glass ·sheets must be evacuated to a high level, and this level of vacuum maintained over the life of the glazing. Fourth, in order to reduce radiative heat flow through the glass, a low emittance coating is applied to the inner surfaces of one or both sheets of glass. In order to evacuate and seal evacuated glazings, it is necessary to incorporate into their structure a small pump-out tube through which evacuation occurs. The tube is necessary because, after the evacuation and bakeout of the structure, the hole through which the evacuation takes place must be sealed. It is difficult to do this directly on the surface of the glass sheets because the sealing process involves high temperatures which cause stresses in the glass sheets. The end of a small tube can be melted and sealed, however, without the necessity to heat the body of the panel which is being evacuated. Typically a small glass tube is sealed into a hole which passes through one of the glass sheets using low melting point glass (solder glass). It has been found that using solder glass to seal an abraded glass surface results in joints which have a high probability of leakage, due to gas flow in the abraded region of the glass beneath the solder glass. It is therefore common practice to seal the tube onto the smooth unmachined inner surface of the glass sheet in order to make a leak-free joint. However, one of the problems with this, is that cracks occasionally occur in the relatively exposed solder glass seal between the pump-out tube and the glass sheet. The cracks arise because the thermal expansion of the solder glass does not exactly match that of the glass sheet. Tensile stresses therefore build up in the solder glass at the seal which may result in the formation of cracks. One of the essential steps in the manufacture of evacuated glazing is the creation of the low pressure space between the glass sheets; commonly referred to as the evacuation process. The space between the glass sheets is generally evacuated through a small tube located at or near a corner of the panel. Conventional pumping techniques are used to reduce the pressure, and the panel is heated to remove dissolved gases from the surface of the glass. Subsequently the structure is cooled and the tube through which evacuation takes place, the pump-out tube, is melted and sealed to complete the cavity. In some designs, reactive materials, called "getters", are incorporated into the cavity in order to absorb gases which are emitted from the internal surfaces during the lifetime of the panel. One of the problems is the comparatively long period of time necessary to evacuate the panel. The time necessary for evacuation depends on many factors including the dimensions of the pump-out tube, the area of the panel, and the size of the gap between the sheets. Summary of the Invention The invention provides a thermally insulating glass panel, comprising two spaced apart sheets of glass enclosing a low pressure space and interconnected by a glass solder edge seal and an array of support pillars, and further comprising a pump-out tube which penetrates one of the glass sheets, characterized in that the pump-out tube has a radial flange which is cemented to the inner surface of that glass sheet. The flange may be cemented to the interior surface of the glass sheet by the use of solder glass. Also, the pump-out tube may extend from the interior of the panel through one of the glass sheets to a recess in the outer surface of that sheet, such that the tube does not extend beyond the outer surface of the glass sheet. The provision of a flange on the pump-out tube, preferably at the interior end, constrains the solder glass cement. In this way it is possible to achieve a very strong joint which does not suffer from the fracture problems of the prior art. Further, the pump-out tube may be positioned at or near the midpoint along one of the sides of the panel. Using this arrangement, the time required to evacuate the space between the glass sheets can be reduced substantially, by up to half in comparison with the earlier technique where the pump-out tube was positioned at a comer of the panel. Consequently, a panel according to this preferred embodiment of the invention has the advantage of being manufactured in a shorter time than earlier panels as a result of the pump-out tube being positioned away from a comer of the panel, near to the midpoint of one of the sides rather than at a corner. Further, the invention concerns a method of constructing a thermally insulating. glass panel comprising providing two spaced apart sheets of glass enclosing a space, the two sheets being interconnected by a solder glass edge seal and an array of pillars, the method including the step of installing a pump-out tube through one of the glass sheets, and the step of evacuating the space by pumping out through the pump-out tube, characterized in that the step of installing the pump-out tube comprises sealing a radial flange of the pump-out tube to the interior surface of the glass sheet. Brief Description of the Drawings Figure 1a is a sectional view through the edge of a thermally insulated glass panel showing a known pump-out tube arrangement; Figure 1b is a sectional view through the edge of a thermally insulating glass panel showing a pump-out tube arrangement embodying the present invention; Figure 2a is a graph showing the time constant for pressure reduction in panels of various lateral dimensions, as a function of the gap between the glass sheets; and Figure 2b is a graph of the time constant for pressure reduction within evacuated panels of different sizes with a very small gap (0.01 mm) as a function of the position of the pump-out tube along the edge of the panel. The same reference numerals have been used in the drawings to refer to corresponding elements. Best Mode of Carrying out the Invention Referring now to Figure 1a, thermally insulating glass panel 1 comprises two spaced apart sheets of glass 2 and 3 enclosing a low pressure space 4. Glass sheets 2 and 3 are interconnected by an array of support pillars (not shown) and an edge seal 5. A pump-out tube 6 extends through glass sheet 2 from the interior space 4 to the bottom of a well 7 in the exterior surface of glass sheet 2. Tube 6 is sealed to the interior surface 8 of glass sheet 2 by solder glass cement 9. Figure 1b, in contrast, shows part of glass panel 1' in which a pump-out tube 6' has a radial flange 10 on its inner end. Tube 6' is sealed to the interior surface 8 of glass sheet 2 by solder glass 9' deposited between flange 10 and inner surface 8. The solder glass is melted at high temperatures, created during the fabrication process, and flows into the gap formed between flange 10 and interior surface 8 to form a very strong joint. Because solder glass 9' is constrained by flange 10 and inner surface 8, it is not so prone to the formation of cracks due to the differential thermal expansion between the solder glass and the glass of sheets in the pump-out tube. Although the invention has been described with reference to a particular embodiment it should be appreciated that it may be embodied in other ways, for instance flange 10 need not be positioned at the extreme end of pump-out tube 6'. Figure 2a shows the periods of time necessary to reduce the internal pressure in the space between the two glass sheets by a factor of e (approximately 2.72). This period of time is referred to as the "time constant" for pressure reduction within the panel, and is used because the pressure reduction is exponentially related to the time required. Typically about 15 time constants are required to reduce the pressure from atmospheric pressure to an adequately low level when the pump-out tube is located at or near a corner of the panel. The data of Figure 2a are calculated for a pump-out tube of 0.5 mm in diameter x 4 mm long, located near the corner of the glazing and distant by 25 mm from the two adjacent edges. The inventors have theorised that for small gaps, the dominant factor in determining the time constant for pressure reduction is the low conductance for gas flow in the space between the two sheets of glass. The area of the panel can be roughly divided into two regions where the spatial patterns of gas flow are qualitatively different. When a pump-out tube is located near the corner of the panel, then the flow closer the pump-out tube is radially inwards towards the tube over a full 2π radians. At some distance away from the pump-out tube the gas flow in the space between the glass sheets is also directed radially towards the corner, but only over approximately π/2 radians. In the intermediate region a complex of flow pattern exists. The inventors have found that for many designs of glazing with small gaps, the gas flow in the regions slightly removed from the pump-out tube over π/2 radians is the dominant factor which determines the time constant for pressure reduction. The inventors have further found that a decrease in the time required -to evacuate the panel can be achieved by relocating the pump-out tube away from the corner to a position closer to the midpoint of one of the sides, and as required by aesthetic considerations close to the edge. Near the midpoint of one of the sides, the gas flow towards the pump-out tube in regions slightly displaced from the pump-out tube occurs over approximately π radians. The conductance for such gas flow when the tube is close to the midpoint of one side is therefore approximately double that which was found when the pump-out tube is near the corner, and the time constant for pressure reduction is therefore less by a corresponding factor. Figure 2b shows the values of time constants as a function of the position of the pump-out tube along the edge of the glass sheets. The data shown are for a sample with a very small gap of only 0.01 mm, and for a pump-out tube which is located 25 mm from one edge. A reduction of the time constant by a factor of approximately 2 is achieved, as expected by the above analysis. As a result of the reduction in time constant the time necessary to evacuate the panel can be cut by approximately half. This time reduction translates into lower manufacturing costs because of the reduced processing time necessary. For a 1 m 2 sample, with an internal gap of 0.2 mm, evacuated through a pump-out tube 1 mm in diameter and 4 mm long, the time required to evacuate the panel is from about 8 minutes to about 4 minutes, when the pump-out tube is relocated from a region near the corner of the panel to a region near the midpoint of a side.

Claims: A thermally insulating glass panel [ 1'], comprising two spaced apart sheets of glass [2, 3] enclosing a low pressure space [4] and interconnected by a glass solder edge seal [5] and an array of support pillars, and further comprising a pump-out tube [6'] which penetrates one of the glass sheets, characterized in that the pump-out tube [6'] has a radial flange [10] which is cemented to the inner surface of that glass sheet. A panel according to claim 1, further characterized in that the flange [10] is cemented to the interior surface of the glass sheet by the use of solder glass [9']. A panel according to claim 1 or 2, further characterized in that the pump-out tube [6'] extends from the interior of the panel through one of the glass sheets to a recess [7] in the outer surface of that sheet such that the tube [6'] does not extend beyond the outer surface of the glass sheet. A panel according to any one of claims 1 to 3, wherein the pump-out tube [6'] is positioned at or near the midpoint along one of the sides of the panel. A method of constructing a thermally insulating glass panel [1'] comprising providing two spaced apart sheets of glass [2, 3] enclosing a space [4], the two sheets being interconnected by a solder glass edge seal [5] and an array of pillars, the method including the step of installing a pump-out tube [6'] through one of the glass sheets, and the step of evacuating the space [4] by pumping out through the pump-out tube [6'], characterized in that the step of installing the pump-out tube [6'] comprises sealing a radial flange [10] of the pump-out tube [6'] to the interior surface of the glass sheet.

Inventors: CLUGSTON DONALD ANDREW, COLLINS RICHARD EDWARD, TANG JIAN-ZHENG, CLUGSTON, DONALD ANDREW, COLLINS, RICHARD EDWARD, TANG, JIAN-ZHENG
Applicants: UNIV SYDNEY, THE UNIVERSITY OF SYDNEY
Application year: 1994
Main classification: E06B3
IPC codes: C03C27, E06B3